among the peacocks and among the mbuna, but not with each other. From my understanding they are too far apart. BTW I have that mix going in a 125g with no such problems, however, it is almost an all male tank.
Its not impossible to get peacock and mbuna crosses I don't believe, but its highly unlikely if you have both sexes of each. They'll breed (and likely hybridize) with each other before trying the other group.
They can hybridize.
There is far less of a chance if you stock each species with optimum sex ratios. Chances are higher if you have lots of different sex singles.
The popular OB peacock is believed to have derived of peacock x mbuna parentage. There has yet to be seen a naturally occuring Aulonocara species.
The same is true with other Haplochromines. Many can also hybridize with mbuna.
If you have females for each male it should not be a problem. You could however end up with a load of dead Aulonocara as they are a lot softer fish than the mbuna. In the confinment of the tank they could be hammered by the mbuna, especially the more aggressive species.